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The Search for Water Ice on Comets and Asteroids

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The Search for Water Ice on Comets and Asteroids UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS FÍSICAS TESIS DOCTORAL The Search for water ice on Comets and Asteroids La búsqueda de hielo de agua en Cometas y Asteroides MEMORIA PARA OPTAR AL GRADO DE DOCTOR PRESENTADA POR Laurence O’Rourke DIRECTOR Michael Küppers Madrid © Laurence O’Rourke, 2021 UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS FÍSICAS TESIS DOCTORAL La búsqueda de hielo de agua en Cometas y Asteroides MEMORIA PARA OPTAR AL GRADO DE DOCTOR PRESENTADA POR Laurence O’Rourke DIRECTOR Dr. Michael Küppers The Search for water ice on Comets and Asteroids La búsqueda de hielo de agua en Cometas y Asteroides TESIS DOCTORAL LAURENCE O’ROURKE Departamento de Física de la Tierra y Astrofísica Facultad de Ciencias Físicas Universidad Complutense de Madrid A thesis submitted for the degree of Doctor en Astrofisica September 2020 Director: Dr. Michael Küppers Tutor: Prof. David Montes Gutiérrez Dedication I would like to dedicate this thesis to my wife Cristina & our children David and Paulina, and to my parents, Laurence and Rita (neé Curley) O’Rourke, who have both passed away but who would no doubt have been very proud of me for this achievement. Acknowledgements This work has been possible thanks to the great support and help provided by my PhD director Michael Küppers and by my UCM tutor, David Montes Gutiérrez. The published papers presented, provide an excellent overview of the significant collaborations I have had across the planetary science community with nearly 50 different co-authors represented in the papers. Their professionalism, expertise and friendship were vital to ensure that the papers I wrote were at the highest level. I would like to make a special mention for those who I have worked with most closely namely Michael, Dominique, Nicolas, Thomas, Sonia and David. A work like this, done in parallel to my job at ESA, is only feasible through countless hours (days & nights, weekends and holidays) spent away from the family. To my amazingly patient and lovely wife Cristina, my son David and my daughter Paulina, my thanks and love. They have kept my feet on the ground while my mind was in space. My sisters (Ita and Anne), brothers (John, Hughie, James, Tony and Tom), and in-laws (Martin, Declan, Assumpta, Loretta, Elaine, Celine and Teresa) in Ireland and the U.S. have been very supportive of me and have provided good advice throughout all my years working and studying and my thanks go to them also. iii Abstract The presence of water is considered one of the key ingredients for the formation of life on Earth. Indeed, current theories as to the origin of the Earth’s oceans suggest that the water located there originated from both comets and asteroids, with the exact fractional contribution still under discussion. However, water is not unique to the Earth; it has been found hidden deep inside polar craters on the moon, as well as in its fragment OH form contained in moon regolith. It is believed to make up over two thirds of the mass of the giant icy planets Uranus and Neptune. It is prevalent in the form of ice throughout the outer solar system on the moons of planets, on Kuiper Belt Objects, and on comets. In the case of asteroids, although few detections of surface ice have been made, evidence of hydrated minerals (any mineral containing H2O or OH) abound. A search for water ice on the surface and subsurface of comets and asteroids provides not only key information as to its presence and distribution on these primitive bodies, it also contributes to giving a deeper understanding on how the solar system formed, how water was delivered to the Earth by these bodies, and indeed how life emerged as a result. This thesis has as its main goal to search for the presence of water ice on and just below the surfaces of comets and asteroids, estimating its coverage and where feasible characterising its properties. This goal is broken down into two objectives, whereby we search for and characterise ice found on cometary (1st objective) and asteroid (2nd objective) surfaces and subsurfaces. In this thesis, we present a number of publications aiming to address these objectives. To address the first objective, we present two papers; one main paper (Nature 2020) and a supporting paper (Astrophysical Journal Letters 2013). In our main paper, we search for water ice inside 2 cometary boulders lying on the surface of comet 67P/Churyumov-Gerasimenko. We apply multiple data analysis techniques to process multi-wavelength observations produced by the Rosetta orbiter merged with in-situ measurements from the Philae lander. The aim is to identify the ice and to characterise its physical properties, in particular its compressive strength and its porosity. In the case of the supporting paper, we searched for cometary activity around a Main-Belt Comet (an asteroid displaying cometary behaviour e.g. coma, tail), estimated its size, and searched for a water exosphere. To address the second objective, we present three papers; one main paper (Astrophysical Journal Letters 2020) and two supporting papers (Nature 2014 & Astrophysical Journal Letters 2013). These three papers focus on a search for water ice present on the surface/subsurface of asteroids, linked to their relative size. While the main paper studies the case for two of the larger asteroids in the main belt, namely (24) Themis and (65) Cybele, the two supporting papers cover both the largest asteroid – Dwarf planet (1) Ceres – and one of the smallest asteroids (Main-Belt Comet P/2012 T1) which was mentioned above as it supports 1 also the first objective. In all three papers, we searched for an exosphere of water vapour using the Heterodyne Instrument on the Herschel Space Observatory (HIFI). We support our HIFI results with data from other observatories/instruments (VLT/FORS2, Herschel/SPIRE, Subaru/COMICS), as well as apply different models to aid in the understanding of our results. In the case of both of these supporting papers, we also present in this thesis the scientific progress achieved since we published them. The results obtained from the papers presented in this thesis are quite unique. The output from our first paper was that we could confirm the presence of primitive (the chemical state of cometary materials as they began to agglomerate/accrete into macroscopic bodies – see Annex F4) ice inside cometary boulders, lying just below their dusty surface. We determined the softness of this primitive ice as well as derived a porosity equivalent to that measured in the comet as a whole. While water ice is generally exposed on the comet through fractures breaking open the dust covered exterior leading to outbursts and jets, in our case the ice was confirmed to be exposed by movements of the Philae lander itself during its bounce across the surface. For our second paper, a non-detection of an exosphere around asteroids (24) Themis and (65) Cybele led to our calculation of highly sensitive upper limits for the water production rate. We estimated that water ice intimately mixed with the asteroids’ dark surface material would cover <0.0017% (for Themis) and <0.0033% (for Cybele) of their surfaces, while an areal mixture with very clean ice would cover < 2.2%. Based on this low percentage of surface coverage, we disproved the results of a Nature paper (Campins et al. 2010) which proposed a link between 3.1 μm absorption feature and surface ice on asteroids. With our first supporting paper, we were the first to confirm the presence of water ice in the asteroid belt when we discovered in 2013 that Dwarf planet (1) Ceres had an exosphere produced from the sublimation of water ice that covered < 10-7 of its total surface area. In our second supporting paper, we did not detect an exosphere even though the Main-Belt Comet had a coma at the time of the Herschel observation. Our non-detection allowed us to derive sensitive upper limits on the water production rate as well as estimate a water ice surface coverage of 0.2%. We close out the thesis by presenting our conclusions and proposing future work in this area. The presence of water ice on comets and on the largest asteroid in the solar system has been confirmed from the studies performed in this thesis. The physical makeup and processes in play to expose this water ice have been discussed in detail. For asteroids of size <300km, the direct confirmation of surface & subsurface water ice remains elusive however. With the launch of JWST (James Webb Space Telescope) as well as the building of bigger and more sensitive Ground based observatories, new techniques are expected to come on line to push the limits in achieving such detections. 2 Resumen La presencia de agua se considera uno de los ingredientes clave para la formación de vida en la Tierra. De hecho, las teorías actuales sobre el origen de los océanos de la Tierra sugieren que el agua que se encuentra allí se originó tanto en cometas como en asteroides, con la contribución fraccionaria exacta aún en discusión. Sin embargo, el agua no es exclusiva de la Tierra; se ha encontrado escondido en el interior de los cráteres polares de la luna, así como en su forma de fragmento OH contenido en su regolito. Se cree que constituye más de dos tercios de la masa de los planetas helados gigantes Urano y Neptuno. Prevalece en forma de hielo en todo el sistema solar exterior en las lunas de los planetas, en los objetos del cinturón de Kuiper y en los cometas. En el caso de los asteroides, aunque se han realizado pocas detecciones de hielo en la superficie, abunda la evidencia de minerales hidratados (cualquier mineral que contenga H2O u OH).
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